>And once the race was no longer between two old guys, age ceased to matter much to a number of news outfits.

Age never mattered with regards to Trump, but it was a meme with Biden even when he was VP. In terms of press coverage and popular perception it was never a race between two old guys, it was a race between "the old guy" and Trump.

And the press did a lot to shape that perception despite both showing signs of decline. That's the point. The coverage was skewed.